1. Field of the Invention
This invention relates to fluid heaters and more particularly relates to in-line fluid heaters for fluids moving in a conduit where the flow rate of the fluid is subject to variations, or where the temperature of the fluid at the outlet of the heater is subject to cycling variations.
2. Description of the Prior Art
Fluid heaters are used in many applications and for many different types of fluids. For example, there are heaters for water, thermoplastic materials, paints, etc. In the spray coating industry, heating paint or coating materials lowers the viscosity of the paint so that paints having high viscosities, which could not normally be applied with spray coating equipment, can be sprayed. The in-line fluid heater disclosed as the preferred embodiment herein was specifically developed for heating paints. However, the inventive principles used are equally applicable to fluid heaters generally.
In-line fluid heaters of the past generally comprised a fluid passage in heat transfer relationship with a heating element; for example see Krohn et al. U.S. Pat. No. 3,835,294. The heating elements in some heaters were in direct contact with the fluid, and in others the heating element heated the fluid indirectly by heating the structure in which the fluid passage was formed, which structure in turn transferred the heat to the fluid in the passage. In heaters of past design the heating element was positioned with respect to the fluid passage in the heater so as to heat the fluid substantially uniformly for the entire length of the passage.
If the thermal characteristics of the fluid and the flow rate of the fluid to be heated were not subject to variations during operation, some heaters were designed so that the outlet temperature of the fluid achieved the proper value with the heating element having constant power input, and there was no need for any control mechanism. However, if the thermal characteristics of the fluid or its flow rate were subject to variations, then a feedback type control was used to assure that the temperature of the fluid being discharged was within a certain allowable range around a desired value. A temperature sensor monitored the temperature of the fluid being discharged from the outlet of the heater, and a control device responsive to the temperature sensor controlled the heating element.
By use of sophisticated and expensive control devices and heater designs, the temperature range could be held to a very close tolerance over a wide range of flow rates and/or thermal properties. However, in heaters of relatively simple and inexpensive design, certain trade-offs had to be accepted. For example, many heaters used a thermostatic type sensor/control combination to monitor the temperature of the fluid at the outlet of the heater. By "thermostatic type" sensor/control is meant one which turns a heater element on or off in response to some preselected temperature. In heaters using a thermostatic type sensor, the temperature of the fluid at the outlet of the heater, even under constant flow rate and thermal characteristics of the fluid, were prone to steady-state cycling of the outlet temperature between high and low peak-to-peak temperatures. This was due to the on-off cycling of the heating element, on/off differential of the temperature sensor, etc. Also in many heaters of past design, when the heater was initially started, or when the temperature setting was suddenly increased, or when the flow rate of the fluid was suddenly reduced, the temperature of the fluid at the outlet of the heater would overshoot the high steady-state peak cycling temperature. That is, the temperature of the fluid would temporarily exceed the high peak temperature which the fluid would reach under steady-state cycling. Conversely, when the temperature setting was decreased or flow rate of the fluid suddenly increased, the temperature of the fluid at the outlet of the heater would undershoot the low steady-state peak cycling temperature. The temperature of the fluid would fall below the low peak temperature which it would drop to under steady-state cycling.
The cycling of temperature, overshoot and undershoot is caused at least in part by what might be termed thermal lag. This thermal lag is caused by the fact that a finite time is required for a body to change temperature and hence to react to a temperature change. When the heating element is on, the temperature of the fluid is increasing. But when the fluid reaches proper temperature, the sensor requires a finite time to respond to this temperature. Also the heating element requires a finite time to cool down. During this time energy continues to be applied to the fluid. This causes the temperature of the fluid to increase beyond the desired to set temperature. When the heating element has been off and the fluid temperature decreases below the desired temperature, a finite time is required for the sensor to react to this situation and to energize the heating element. The temperature of the fluid continues to decrease before the heating element heats up and causes the temperature of the fluid to increase.
It is an object of the present invention to reduce the steady-state cycling of the feedback controlled fluid heaters as well as their overshoot and undershoot characteristics. Through the present invention these reductions can be achieved in simple inexpensive heaters using thermostatic control, as well as in heaters using more sophisticated control means, and without adding undue cost to the heater.